Surface roughness analysis of multilayer x-ray optics

Rigaku Innovative Technologies (RIT) produces x-ray optics based on multilayer mirrors. A multilayer mirror is deposited on a wafer and mounted on a solid backing plate in an elliptical shape to focus x-rays. The wafer surface imperfections, defects from the multilayer deposition, and figure errors induced by the mounting process result in some focal spot widening for the final optics. An AFM is used in the spatial period range 0.1 - 10 microns, and a "ZYGO" interferometric microscope is used in the spatial period range I micron - 5 mm, to study these imperfections determining the influence of each technological step oil the focal spot quality. AFM analysis shows dramatically different roughness between I x I micron and 20 x 20 micron field of view on super-polished substrates from some suppliers and only a little difference from others. A smoothing effect of a multilayer coating at spatial periods less than one micron as well as defects in multilayer coatings have been observed with power spectral density analysis. Machining marks on the surface of wafers are clearly seen at ZYGO microscope pictures. Ray-tracing simulations based on the ZYGO data show the focal spot shape changes due to the figure errors introduced at the step of a multilayer coated wafer mounting and only background scattering with no focal spot widening from defects induced at the step of multilayer deposition.